The present invention relates to an electronic tuning system, and more particularly, to an electronic tuning system which comprises an electronic tuner for tuning to radio waves on a broadcast channel of television broadcasting, radio broadcasting and the like, or a local channel used in a variety of radio devices.
As is well known, in the television broadcasting and radio broadcasting, respective broadcasting stations are assigned radio waves in different frequency bands from one another as broadcast channels. Thus, on the receiver side, radio waves on a desired broadcast channel are selectively received to enjoy contents of broadcasting provided by a desired broadcasting station.
Generally, a receiver for receiving radio waves on a particular channel is configured in the following manner. A received signal received through an antenna is mixed with a local frequency signal supplied from a local oscillator by a mixer. A signal generated from the sum or the difference of the two signals by a mixer is supplied to an intermediate frequency filter. The intermediate frequency filter selectively supplies a detector circuit only with a signal having an intermediate frequency. The detector circuit demodulates the received signal corresponding to a predetermined broadcast channel.
The receiver comprises an electronic tuner for selecting a desired broadcast channel. The electronic tuner adjusts (tunes) a frequency band of the local frequency signal supplied from the local oscillator such that only the received signal on the desired broadcast channel passes through the intermediate frequency filter. The electronic tuner has a voltage controlled oscillator (VCO) including a local oscillator, and controls a voltage applied to the voltage controlled oscillator to control the frequency band of the local frequency signal output from the oscillator.
In an electronic tuning system comprising an electronic tuner, channel selection information on a broadcast channel selected through a voltage controlled oscillator (frequency information or tuning control information) is stored in a memory, such that the channel selection information stored in the memory is read upon next channel selection to permit prompt tuning to the broadcast channel. For this reason, the channel selection (tuning control) and writing the channel selection information into the memory have been previously performed for a plurality of broadcast channels, so that the selection of an arbitrary broadcast channel can be instantaneously made through an extremely simple operation of reading such channel selection information from the memory.
Generally, memories used in such electronic tuning systems often require a sustain current for a RAM (Random Access Memory) or the like. Therefore, when such an electronic tuning system is equipped in a portable device driven by a battery such as a dry battery, the following disadvantages will result.
Specifically, a device which employs a dry battery as a power source such as a portable device essentially involves replacement of dry batteries when the device is used for a long time. However, the replacement of batteries results in interruption of the sustain current, so that information stored in a memory is erased. In such a situation, after the replacement of batteries, the user is forced to perform complicated operations which involves selecting (tuning) each broadcast channel and writing the channel selection information into the memory.